1. Field of Invention
The invention relates to characterizing communication channels.
2. Prior Art
In an orthogonal frequency division multiplex (OFDM) receiver, a real-time complex signal is typically converted to the frequency domain using a Fast Fourier Transform (FFT), recovering the orthogonal sub-carriers in the process. The recovered sub-carriers are then corrected with channel equalization, and frequency and time compensation. The sub carriers are demodulated to recover the data modulated onto the sub-carriers by the transmitter.
Characterization of the communication channel is needed to estimate the impairments to the transmitted signals passing through the channel. Channel response, also called channel transfer function, is the actual signal distorting characteristics of the communication channel. Channel estimation is the process of determining the channel response. Impulse response is the time domain version of the channel response. One characteristic of particular interest is the channel echo profile, the result of multiple signal paths due to signal reflections caused by the channel.
A known data sequence is transmitted through the channel that can be used by the receiver to compute the channel response.
It is common practice in communication systems to use time domain methods to compute the channel transfer function, which is represented by the channel impulse response. Computation of channel impulse response by conventional methods requires performing correlation between the received signal and the known sequence, which is costly to perform in software. Time domain cross-correlation techniques require computations that increase significantly with the channel length and sequence length, requiring computations proportional to the product of channel and sequence length. A large amount of data memory is required in the receiver to hold the received sequence during processing.